Pin images used for the cel release pipeline

[AlanGreene]

Changes

https://github.com/tektoncd/plumbing/issues/2197 /kind misc

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ ] Includes docs (if user facing)
• [ ] Commit messages follow commit message best practices
• [ ] Commit messages includes a project tag in the subject - e.g. [OCI], [hub], [results], [task-loops]

See the contribution guide for more details.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

Approval requirements bypassed by manually added approval.

This pull-request has been approved by:

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - **[cel/OWNERS](https://github.com/tektoncd/experimental/blob/main/cel/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment